From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 12E8FC433E1 for ; Mon, 25 May 2020 15:20:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E4E5F2071A for ; Mon, 25 May 2020 15:20:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="T8I/C9fD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2404108AbgEYPUb (ORCPT ); Mon, 25 May 2020 11:20:31 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:27177 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S2403999AbgEYPUb (ORCPT ); Mon, 25 May 2020 11:20:31 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1590420029; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=HYjKBsVG8/aQRDDWxnpTEgM4JQmNET1Q9YOObHs51wo=; b=T8I/C9fDoU/ROi3tZkI3ciQdHfZQXLRq0RDv/pNCdqsv3GLoFgJMX2p4ii5v7uckugTq3k U1hkpCit2aXvvHJI8DxS7BKk140wNFSlBnrJchLYEqKJJFpjA0MDL4rHFvgl3gkYNnxeaH NObu1xhLpdxYxD5cMMJcu/zvMGwc4Nw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-491-c78kMZeNNO2R3tqKxuLtLA-1; Mon, 25 May 2020 11:20:27 -0400 X-MC-Unique: c78kMZeNNO2R3tqKxuLtLA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 92B0B80183C; Mon, 25 May 2020 15:20:24 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-112-121.ams2.redhat.com [10.36.112.121]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8A4AB5798D; Mon, 25 May 2020 15:20:21 +0000 (UTC) From: Florian Weimer To: Mathieu Desnoyers Cc: libc-alpha , Rich Felker , linux-api , Boqun Feng , Will Deacon , linux-kernel , Peter Zijlstra , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19) References: <20200501021439.2456-1-mathieu.desnoyers@efficios.com> <20200501021439.2456-2-mathieu.desnoyers@efficios.com> <87v9kqbzse.fsf@oldenburg2.str.redhat.com> <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> Date: Mon, 25 May 2020 17:20:19 +0200 In-Reply-To: <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Mon, 25 May 2020 10:51:45 -0400 (EDT)") Message-ID: <87367ovy6k.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers: > The larger question here is: considering that we re-implement the entire > uapi header within glibc (which includes the uptr addition), do we still > care about using the header provided by the Linux kernel ? We don't care, but our users do. Eventually, they want to include and to get new constants that are not yet known to glibc. > Having different definitions depending on whether a kernel header is > installed or not when including a glibc header seems rather unexpected. Indeed. > *If* we want to use the uapi header, I think something is semantically > missing. Here is the scheme I envision. We could rely on the kernel header > version.h to figure out which of glibc or kernel uapi header is more > recent. Any new concept we try to integrate into glibc (e.g. uptr) > should go into the upstream Linux uapi header first. I think we should always prefer the uapi header. The Linux version check does not tell you anything about backports. > For the coming glibc e.g. 2.32, we use the kernel uapi header if > kernel version is >= 4.18.0. Within glibc, the fallback implements > exactly the API exposed by the kernel rseq.h header. Agreed. > As we eventually introduce the uptr change into the Linux kernel, and > say it gets merged for Linux 5.9.0, we mirror this change into glibc > (e.g. release 2.33), and bump the Linux kernel version cutoff to 5.9.0. > So starting from that version, we use the Linux kernel header only if > version >= 5.9.0, else we fallback on glibc's own implementation. Fortunately, we don't need to settle this today. 8-) Let's stick to the 4.18 definitions for the fallback for now, and discuss the incorporation of future changes later. >>> +/* Ensure the compiler supports __attribute__ ((aligned)). */ >>> +_Static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); >>> +_Static_assert (__alignof__ (struct rseq) >= 32, "alignment"); >> >> This needs #ifndef __cplusplus or something like that. I'm surprised >> that this passes the installed header tests. > > Would the following be ok ? > > #ifdef __cplusplus > #define rseq_static_assert static_assert > #else > #define rseq_static_assert _Static_assert > #endif > > /* Ensure the compiler supports __attribute__ ((aligned)). */ > rseq_static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); > rseq_static_assert (__alignof__ (struct rseq) >= 32, "alignment"); Seems reasonable, yes. __alignof__ is still a GCC extension. C++11 has alignof, C11 has _Alignof. So you could use something like this (perhaps without indentation for the kernel header version): #ifdef __cplusplus # if __cplusplus >= 201103L # define rseq_static_assert(x) static_assert x; # define rseq_alignof alignof # endif #elif __STDC_VERSION__ >= 201112L # define rseq_static_assert(x) _Static_assert x; # define rseq_alignof _Alignof #endif #ifndef rseq_static_assert # define rseq_static_assert /* nothing */ #endif rseq_static_assert ((rseq_alignof__ (struct rseq_cs) >= 32, "alignment")) rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment")) And something similar for _Alignas/attribute aligned, with an error for older standards and !__GNUC__ compilers (because neither the type nor __thread can be represented there). Thanks, Florian